As known, gas masks intended for the above-described use comprise substantially a facepiece made of impermeable material, resistant to chemical agents and associated with a harness which allows the mask to be put on the user's head so as to provide a tight seal between the edges of the facepiece and the user's face.
Once the mask is put on, the user can inhale air from the outside through an inflow opening arranged on the facepiece and provided with a threaded fitting on which is assembled a filtering element intended to decontaminate the air being inhaled. The air subsequently exhaled by the user is expelled from the mask through an outflow opening, also arranged on the facepiece and provided with a respective single-acting valve.
The facepiece comprises, moreover, two eye-pieces or, alternatively, a single transparent screen to give the user full visibility. To prevent the air being breathed out from causing clouding of the eye-pieces, in many cases, provision is made of a so-called "half mask," associated with the facepiece and communicating with the outside through the outflow opening and the single-acting valve.
The half mask allows the air breathed out to be directly discharged outside without filling completely the inner portions of the facepiece, in particular those corresponding to the eye-pieces. Still, in order to prevent a possible clouding of the eye-pieces, it is foreseen that, in the inhalation phase, the air flow inhaled through the filter may be guided into the inner portions of the facepiece so as to skim the surfaces of the eye-pieces and to be then sucked inside the half mask through further check valves.
In this way, also, the clouding of the eye-pieces due to the user's perspiration is significantly reduced.
Generally, the facepiece includes also a phonic cap, functioning to transmit outside the user's voice to prevent an excessive muting of the same owing to the presence of the facepiece.
In masks of modern design, both the inflow and the outflow openings are obtained in a so-called "nozzle," tightly engaged in the lower portion of the facepiece and housing also the phonic cap. More particularly, the inflow opening is provided in the upper part of the nozzle and flows almost directly, by means of a short connection duct, into the interior of the facepiece, at the base of the eye-pieces and in a median position between them.
The outflow opening is instead provided in the lower part of the nozzle and opens directly outside the mask, below the inflow opening. The phonic cap is situated over the inflow opening and is directed toward a chamber placed behind the connection duct and communicating with the outlet of the outflow opening downstream of the corresponding single-acting valve.
It has been noted that the arrangement of the above-described elements yields certain inconveniences regarding the practical and functional employment of the gas mask. At first, it can be seen that the position of the inflow opening involves a corresponding position of the filtering element which is not quite appropriate in view of a rational distribution of the mask portion. In fact, the filtering element, which is situated in the upper part of the nozzle and has a considerable weight, may provide high moments of inertia opposing movements of the user's head. In this situation, a comfortable use of the mask is compromised. Further, the position of the filtering element limits the user's downward field of view.
It was also noted that the position of the inflow opening limits the possibility of building up the facepiece in such a way that the eye-pieces are sufficiently near the user's eyes to permit the use of binoculars, microscopes or optical instruments in general, when the mask is worn.
Moreover, the direct connection between the inflow opening and the interior of the facepiece can cause some problems if the mask is to be used at low temperatures. In fact, in these circumstances, the inhaled air cannot be heated before skimming the user's face, giving, therefore, rise to a significant discomfort level for the user.
Furthermore, the entrance of air at the base of the eye-pieces, in a median portion between them, does not represent an ideal condition to obtain an optimum distribution of the air flow inside the facepiece. This can result in an irregular de-clouding of the eye-pieces. It is also to be noted that the air skimming the surface of the eye-pieces is not heated at all, so that it is not in the best condition to yield an efficient de-clouding function.
Another disadvantage of the prior art is that the air coming out from the outflow opening tends to skim the surfaces of the filtering element. At low temperatures and after a prolonged use of the mask, this circumstance can cause formation of a certain amount of ice on the filtering element, due to freezing of the water molecules present in the exhaled air. The consequent increase in the weight of the filtering element compromises therefore the comfort, and above all the safety, of the mask.
The position of the phonic cap, situated behind the inflow opening, affects significantly a good transmission of the user's voice toward the outside. In fact, the user's voice is compelled to reach the outflow opening and to come out below the filtering element. To compensate for the poor quality of this transmission, it is at present necessary to provide large-sized phonic caps, to the detriment of the space available for the single-acting valve associated with the outflow opening.
In this condition, the diameter of the single-acting valve must be considerably reduced, with a consequent increase in the effort required by the user to expel air from the mask. Alternatively, the single-acting outflow valve can have an elongated configuration, involving, however, high production costs and giving results, as to an efficient and reliable employment, which are anyhow worse than those obtainable by the use of circular valves of appropriate diameter.
In the production of masks according to the conventional technique, some problems arise from the contrasting needs of providing facepieces which must be sufficiently stiff to prevent excessive oscillations of the unit formed by the nozzle and the filtering element and which must be, at the time, sufficiently soft and elastic to ensure a perfect sealing action around the user's face.
Generally, these objectives are satisfied by making the facepiece with soft rubber layers having a reduced thickness along the edges to be sealed around the user's face and a greater thickness, to provide additional stiffness, in the front portions of the facepiece. These expedients, however, result in a significant increase in the overall mask weight and, therefore, may compromise its comfort, in particular in view of a prolonged use.
Attempts to eliminate such disadvantages have led to the production of facepieces whose front portions are reinforced with fabric layers embedded in the rubber. However, this requires long and complicated procedures, not suitable for mass production.
Also, masks were built up, whose facepieces comprise two portions stuck together. A first portion, which extends around the edges of the facepiece, functions to ensure a seal around the user's face and is therefore made of soft rubber, and the remaining portion of the facepiece is made of stiffer rubber. This solution involves problems regarding reliability of use, since the mask may be employed in atmospheres containing solvents able to attack chemically the bonding agents used to join the two portions forming the facepiece.